SPID#: 54 Over the last decade new research has offered the strong possibility that some aggressive behavior may be modulated by brain neurotransmitters. Recent research using human subjects has demonstrated a relationship between psychopathological syndromes, including excessive aggressive and impulsive tendencies and low 5-HIAA levels. The purpose of this study is to assess the relationship between serotonin metabolite (5- hydroxyindoleacetic acid, 5-HIAA) concentrations in cerebrospinal fluid (CSF) and behavior in two species which exhibit aggression at similar rates but distinctly different degrees of severity. Subjects from each group were studied from September-November 1995 and will be studied from March-May 1996 to include the rhesus monkey's breeding season and nonbreeding season. From September-November 1995 behavioral data was collected on each of the groups concurrently and a CFS sample was collected at the mid point of the three month sample period. The hypothesis to be tested is that sooty mangabeys exhibit less serious aggression than rhesus macques and that increased serious aggression will be reflected in lower concentrations of 5-HIAA. It is also anticipated that within-group variability will exist and will be related to such other positive behaviors as grooming and reconciliation.